Artifacts with Anisotropic Shader
The Anisotropic shader works using a Tangent to control in which direction the reflections get squashed.

If you don't specifically tell it which tangent to use, it'll compute a tangent from the geometry itself. I assume because your model is perfectly flat, it had a bit of trouble doing this predictably.

So instead, pick one of your UV maps and plug that into the Anisotropic shader's Tangent input:



The reason those artifacts appeared to come from geometry that doesn't exist is because you're using an Explode Modifier that modifies the mesh.